Reset mixer and element before Refresh

This fixes an issue when Refresh fails but the mixer or element is still
pointing to the old address.

Bug: b/201413038
Test: None
Change-Id: Ic7f8d88d1c4c911e81756785455a84ec92c6c814
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3739406
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Mingyu Kang <kenkangxgwe@google.com>
Cr-Commit-Position: refs/heads/main@{#1019905}

chromecast/media/cma/backend/alsa/scoped_alsa_mixer.cc

@@ -41,7 +41,8 @@ void ScopedAlsaMixer::Refresh() {
     LOG(INFO) << "Opening mixer element \"" << mixer_element_name_
               << "\" on device \"" << mixer_device_name_ << "\"";
   }
-
+  mixer = nullptr;
+  element = nullptr;
   int alsa_err = alsa_->MixerOpen(&mixer, 0);
   if (alsa_err < 0) {
     LOG(ERROR) << "MixerOpen error: " << alsa_->StrError(alsa_err);